It's midday when we finally arrive. Scrambling down a steep path shaded by giant hopper trees and thickets of prickly rattan, we reach a small lake fed by underwater caves and flushed jade green. Karst mountains, their sheer limestone sides alive with lush splays of fern, surround the lake, their white peaks stark against the royal blue sky.

A clutch of bamboo houses adorned with white and purple orchids spilling out of coconut shells float on one side of the lake. We pay a woman, missing front teeth and wearing an "I Love Kansas" T-shirt, to take us to the lake's far side on a raft made from bamboo poles.

Leaving the raft, we clamber a few metres up the side of a karst mountain and into Tham Pakarang, or Coral Cave, a gaping black mouth cut into the side of the cliff. Inside are stalagmites and stalactites, 20 million years old, rarely visited, and each in pristine condition.

Our guide, Khun U, a spritely young Thai man who grew up nearby, leads my husband and I deep into the hollow with a battery-powered light. Columns dangle from the ceiling to the floor like organ pipes. Others huddle on the cave's sides, their crystal-like surfaces glistening in the torch light like thousands of tiny diamonds. Some look like coral; others like Venetian blinds.

Those who know Khao Sok call it Thailand's best-kept secret. The 739-square-kilometre national park blanketed by thick rainforest in southern Thailand is halfway between the resort island of Phuket and the city of Surat Thani, the gateway to Koh Samui.

Khao Sok was established in the 1980s when the government cleared the area of small subsistence villages and flooded it for a hydro-electric dam called Chieow Laan. Khun U's father was born in a village in the mountains, an eight-hour hike away from the nearest road. His family earned their living by poaching animals for illegal wildlife trade.

Khun U's father now leads a conservation program and works with scientists to identify rare flora and fauna that still roam in the forest, including clouded leopards, Great hornbills that nest in the cliff faces, and rafflesias, the world's largest flowers, measuring one metre in diameter.

Not many tourists venture this way. The majority of visitors to this area are Thai groups and families who come to picnic on giant gourami - a flaky-fleshed fish that they deep-fry and top with piquant sweet-chilli sauce - and take wooden long boats out on the stunning lapis lazuli-blue waters of Chieow Laan. Part of the reason visitors are mostly local is that, until the opening of Thanyamundra Organic Resort - an up-scale nine-room retreat - in early 2012, there were only backpacker homestays to sleep at.

Thanyamundra was established by a former banker, German Klaus Hebben, as a weekend retreat from his businesses in Phuket. Hebben also built an international school in Phuket when he couldn't find a suitable education for his children. His next project was Thanyapura, an "integrative centre" in Phuket whose $100 million services include Olympic-standard training facilities for athletes, a mind centre to "cultivate mental balance", a medical clinic and two hotels.

When Hebben built Thanyamundra he added an organic garden to supply both his kitchen and resorts in Phuket eager for traceable naturally grown herbs and vegetables. Less thought went into the location.

"Klaus called me one day and said, 'Hey, why don't you come to Thailand to start an organic farm for me?"' says Pierre Larigaldie , the French farmer who oversees Thanyapura's garden. "So I came and we tried. I didn't realise it would be so hot and wet here."

Larigaldie, a cordial sixtysomething with a hearty sense of humour, shows me through his vegetable patch, which was recently inundated by several inches of unusually hard monsoonal rain. Many of the vegetables have died under the strain, but a few varieties are still clinging on, including wild rocket, Slim Jim eggplants, Malabar spinach and small-leaf Italian basil.

Larigaldie says the garden has been a learning curve, but they have mastered the art of brewing effective micro-organisms using ripe fruit for both fertiliser and insecticide - the staff have taken a liking to the odd sip, believing it wards off illness and parasites. What is ready to harvest heads to the kitchen for our lunch: a spicy green curry.

If there were such a thing as an ideal resort, this could well be it. Clearly costing a lot more to build and run than the room rates request, Hebben has excelled in merging peace, intimacy and comfort into a hotel.

The nine guest rooms are spacious and smart, with reproduction vintage and antique furniture, and most have balconies commanding superb views over the veggie garden and a verdant forest-lined valley ringed by karst outcrops. There are no plastics - aside from drinking bottles, which staff assure will be replaced by an on-site water treatment plant soon - no chemicals and no energy-hungry gadgets, apart from airconditioners.

A 50-metre swimming pool presides over the gardens and there is a gym, a small massage room and a flock of mountain bikes for exploring the surrounding countryside.

But what really sets Thanyamundra apart is the staff. Overseen by a former Aman manager, South African Shaun Dunhofen, they have been drawn from the big luxury resorts in Phuket

and are skilled in the art of polite chattiness and anticipating needs; remarkable for any hotel, let alone one in the middle of woop-woop Thailand.

No task is too big for them and, with so few guests, it's easy for them to go out of their way to do special deeds.

On our final night, we are led down the garden path to a wooden and thatched pavilion for dinner. Staff have decorated the sides and roof with flowers and leaves from the garden, and a small table in the centre with ribbons and candles. Bamboo torches light up the banana trees opposite.

We sit on rectangular cushions and are served a glass of wine followed by banana blossom salad and freshly grilled fish in Thai spices. Staff struggle to see in the pitch-blackness and whisper and laugh as they fall over bushes while trying to serve food being cooked by a chef 20 metres further into the garden.

They later tell me it was all their idea. At no extra charge to the guest, they just like to spoil.

It's difficult to know what the better find is: Khao Sok National Park or its latest arrival, Thanyamundra Organic Resort.

Leisa Tyler was a guest of Khao Sok's Thanyamundra Organic Resort.

FAST FACTS

Getting there

Singapore Airlines has a fare to Phuket for about $1180 return from Sydney and Melbourne including taxes. Fly non-stop to Singapore (about 8hr) and then non-stop to Phuket (1hr 45min with Silkair); phone 13 10 11, see singaporeair.com. Khao Sok is a two-hour drive from Phuket airport.

Staying there

Thanyamundra has nine rooms available from 10,500 baht ($419) double occupancy, including breakfast. See thanyamundra.com.

Touring there

Thanyamundra can arrange full-day tours with Khun U to Khao Sok for 6000 baht. Independent travellers can pick up boats and guides from the park's front entrance. Nobody is permitted to enter the caves without an accredited guide. See dnp.go.th.